(a) Field of the Invention
The present invention relates to lifting mechanisms whose load engaging member is adapted to raise weights and, more particularly, to a load dependant, variable displacement jacking mechanism providing at least 3 shifts in performance characteristic relating to the effort expended by the operator and how quickly the lift mechanism and/or the load is raised. Each of the 3 performance characteristics form an operator felt response to the act of raising the lift arm and the load.                the first shift/response is one of minimal handle pumping effort as the operator raises a substantially unloaded lifting member quickly to contact the load;        the second shift/response is one of greater than minimal pumping effort as the user raises a considerably less than rated capacity load at a slightly slower raising speed, but significantly more rapid than if the load were rated capacity;        the third shift/response is one of minimal operator effort to raise a significantly greater load but at slower raising speed than the previous response.        
(b) Description of the Prior Art
A lifting mechanism using a conventional hydraulic device for use in a vehicle jack or the like is generally comprised of a valve block containing check valves and passages communicating to a ram piston apparatus and a pump piston apparatus. The pump piston is actuated by a means to suck in hydraulic oil and then to pump hydraulic oil through passages and valving into a lifting tube, causing the ram piston to extend, thereby acting upon the lifting mechanism, causing it to rise. This device has limitations due to the felt response of the operator to the pumping action of the actuator means, i.e. a single slow raising speed whether the lifting mechanism is loaded or unloaded so that the user employs much effort to repeatedly reciprocate the pump piston and the operator expends much actuator pumping effort when the lifting mechanism finally contacts the load. There are known, dual-speed lifting mechanisms that enable the ram piston apparatus to engage the load quickly from a fully retracted lift mechanism position and then continued pumping results in raising the load at a slow speed. However, these dual speed mechanisms have no means of enabling the operator to raise substantially lesser that rated capacity loads quickly, promoting both poor lift performance and operator fatigue. Therefore, it is desirable to provide three stages of felt response in order to eliminate the drawbacks of the conventional lifting mechanism.